Fiber-reinforced composite members constituted by lightweight, high-strength carbon-fiber-reinforced plastics (CFRPs), etc. are widely used for structural members for aircrafts, etc. To connect fiber-reinforced composite members to each other, or fiber-reinforced composite members and metal members, each member is bored and riveted. Twist drills having chip-discharging spiral grooves from tip ends to shanks are conventionally used to bore the fiber-reinforced composite members. However, cutting is difficult in the fiber-reinforced composite members containing reinforcing fibers. Boring with a twist drill leaves burr on a cut surface and fibers projecting from the cut surface. Also, vibration during drilling causes delamination from the cut surface, resulting in strength decrease and fatigue failure. Accordingly, pluralities of twist drills with different diameters are used successively from a smaller-diameter one to expand the diameters of holes, and finishing is conducted by a reamer, resulting in large numbers of steps.
As a drill for boring stacked composite member and metal member without causing delamination in the composite member, JP 63-306812 A proposes a twist drill comprising first blades slanting at a large angle such that their gaps widen as going rearward from a tip center, and second blades continuous to the first blades and slanting at a smaller angle, in a tip end portion.
As a twist drill for boring CFRP without burr and projecting fibers, JP 2-237709 A proposes a twist drill comprising a tip blade consisting of a candle-point-type, inner part and a fishtail-type outer part, which are connected in a V shape.
JP 2001-293604 A proposes a method for boring a composite by a rotating tool, the tool having blades having a circular locus having a desired diameter. This method can be conducted with fewer cutting tools, resulting in a simpler cutting step, generating fewer chips while preventing burring and delamination.
However, the boring of CFRP using the twist drills described in JP 63-306812 A and JP 2001-293604 A or by the method described in JP 2001-293604 A would fail to provide holes with sufficiently high dimensional accuracy and roundness and low surface roughness. With holes having poor dimensional accuracy and roundness, riveted portions are vulnerable to displacement, and cracking likely occurs from rivet holes when a large load is applied to the riveted portions. As a result, high strength and durability cannot be obtained in riveted portions between fiber-reinforced composite members, or between fiber-reinforced composite members and metal members. To increase the dimensional accuracy and roundness of holes, reaming, etc. are needed, inevitably resulting in higher production cost.
Known as tools capable of conducting drilling and reaming simultaneously are burnishing drills having drilling blades and reaming blades. However, the burnishing drills are generally used to bore metal members, but there are no precedents of using the burnishing drills to bore fiber-reinforced composite members.